Year 421 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Vibulanus and Barbatus (or, less frequently, year 333 Ab urbe condita). The denomination 421 BC for this year has been used since the early medieval period, when the Anno Dominicalendar era became the prevalent method in Europe for naming years.

Nicias, the leader of the aristocratic and peace party in Athens and Pleistoanax, King of Sparta, negotiate the Peace of Nicias between Athens and Sparta, which brings a temporary end to the Peloponnesian War. The essence of the Peace of Nicias is a return to the prewar situation: most wartime gains are to be returned. Seventeen representatives from each side swear an oath to uphold the treaty, which is meant to last for thirty years or one generation (meaning they are not responsible for the next generation's decision). All of Sparta's allies agree to sign the peace, except for the Boeotians, Corinth, Elis, and Megara.